The transcriptional events associated with the development of the erythropoietic spleen in mice are currently being studied with the support of this grant. Following a phenylhydrazine-induced hemolytic anemia, there is an early increase in the production of cytoplasmic RNA and specific activity of high and low ionic strength nuclear RNA polymerase. The soluble chromatin isolated from spleens at the peak of the polymerase activity, has increased template activity with C. coli RNA polymerase compared to chromatin isolated from preanemic or late erythroid phase spleens. This chromatin also demonstrates a quantitative difference upon centrifugation through a magnesium containing sucrose gradient such that a considerable fraction of template activity is slowly sedimenting. The magnesium induced aggregation of nonsheared interphase chromatin was studied further and it was found that the fractions precipitated at low cation concentrations contained DNA duplexes of between 5 minus 10 times 10 to the 6th power daltons, while the soluble fractions were between 1 minus 2 times 10 to the 6th power and the latter fractions demonstrated decreased hyperchromicity during thermal denaturation. The fractional precipitation of interphase chromatin with magnesium is currently being studied.